One of the key functions of planned changes to version R5 of the UMTS (Universal Mobile Telecommunications Systems) standard is the introduction of an HSDPA (High Speed Downlink Packet Access) time-shared access radio channel for high-speed downlink transmission of data packets. The paper “High Speed Downlink Packet Access (HSDPA)—Enhanced Data Rates for UMTS Evolution” by I. Forkela, H. Klennera, and A. Kempera published in Computer Networks, Volume 49, Issue 3, 19 Oct. 2005, pages 325-340, provides a general description of integrating HSDPA into the UMTS standard. This type of access increases cell capacity and offers higher data rates than UMTS version R99 on the downlink radio connection between a base station (Node B) and mobile terminals. An HS-DSCH (High-Speed Downlink Shared CHannel) downlink shared access radio channel is time-shared between mobile terminals in a given cell by a scheduling device that selects a receiver mobile terminal for a given time slot. The data packets transmitted to the mobile terminals are time-division multiplexed by a transmitting station, each terminal receiving in turn data packets transmitted during time slots with a duration of the order of one millisecond assigned to respective terminals. For opportunistic scheduling, for example, the scheduling device elects to transmit during a given transmission time slot to a mobile terminal for which radio conditions are favorable. To this end, the scheduling device uses radio measurements of a reference radio signal for the cell carried out by the mobile terminals and sent back to the Node B. This technique can handle rapid changes in the quality of the radio link between the Node B and a mobile terminal. It also adapts the modulation and coding formats used for transmission as a function of radio link quality and uses a specific retransmission protocol known as the Hybrid Automatic ReQuest (HARQ) protocol.
Also, a mobile terminal may find itself in an area of overlap between first and second cells and connected either to the transmitting station of the first cell or the transmitting station of the second cell. These transmitting stations can belong to two different Nodes B or to the same Node B having two radio antennas covering two separate angular sectors. According to version R99 of the UMTS standard, the mobile terminal can maintain a plurality of simultaneous radio links with a plurality of respective transmitting stations in order to facilitate passage from one cell to the other. This technique, known as “soft handover”, further increases the power of the signal received by the terminal in areas of high interference, such as areas of overlap between two cells, as the various transmitting stations engaged in the soft handover process transmit the same data to the mobile terminal simultaneously.
It is not possible to transpose the soft handover technique to HSDPA since scheduling is managed independently at each transmitting station and as a function of scheduling criteria specific to that station.
There is therefore a need for a technique for increasing the power of the signal received by a mobile terminal in areas of overlap between two or more cells that retains the advantages of using a downlink radio connection with time-shared access in a cell.